Cycle Chem Sat Class

7/30/2019 Cycle Chem Sat Class

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M

U

CYCLE

CHEMISTRY


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NEED OF CYCLE CHEMISTRY

To save the steam-water circuit from

corrosion i.e., to minimize corrosion

Thereby reducing occurrence of

BTF

Condenser leakage

To save turbine blades from deposition


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Typical drum water plant cycle showing locations

of impurity ingress, corrosion and deposition

Chemical

feed

Boiler

HP

Turbine

HP Heater

Attemperator

Dearator

LP Heaters

IP Turbine

LP

Turbine

Condenser

M

U

CEP

CPU


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WHAT IS CRUD

Corrosion Residue Under Deposit

Water (in presence of NH3 CO2, D.O.)

under different operating conditions reacts

with Fe, Cu and other metallurgy in the

system continuously leading to the

accumulation of corrosion products knownas CRUD


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The process of corrosion/ errosion accelerates in

presence ofCRUD

However, in the boiler after economiser chances of

corrosion is less due to the protective magnetite layer.The most vulnerable portion is pre-boiler section

because.

Though the system has oxide coating but this coating is

quite porous, which facilitates oxygen attack leading topitting.

HOW CRUD FORMATION IS INEVITABLE:


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CYCLE CHEMISTRY AIMS TO REDUCE

Flow accelerated corrosion to minimum

possible.

To minimise the amount of iron andcopper at eco. Inlet (< 2 ppb).


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Function of CPU

removes dissolved solids from feed water

(sodium, silica, sulphate ,chloride etc.) by

ion-exchange method in an extremely lowlevel

removes suspended solids from feed water(crud of ferric oxide and other impurities) by

filtration


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CPU OUTLET

PRESSURE

EQUALISATIONLINE

INLET LINEFROM CEP

DISCHARGE

MANHOLE

BOTTOM

SCREWRESIN TRANSFER

LINE

CONDENSATE POLISHING

UNIT

BOTTOM PLATE

CONDENSATE POLISHING UNIT

FLOW DIAGRAM OF CPU


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FEED WATER CHEMISTRY

LP Dosing

BOILER WATER CHEMISTRY

HP Dosing


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THERE ARE TWO TYPES OF

FEED WATER TREATMENTUSED AT VSTPS:


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LP DOSING

AVT R - All Volatile Treatment-Reducing

Media

It is classical (AVT) using ammonia and a reducing agent

or oxygen scavenger (such as hydrazine). Thisprovides a reducing environment(ORP N2 +2H2O

else 3N2H4 ----> 4NH3 + N2 (in boiler)


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AVT O- All Volatile Treatment-OxidizingMedia

It is the same as AVTR minus the reducing agent.

This provides an oxidizing environment(ORP>0 MV)

LP DOSING


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THEN WHY WE USE AVTR

In copper based metallurgy the Cu pickup

by ammonia is more in oxidising media

Thus in Stage I -210MW units- AVTR

In Stage II and III-500MW units- AVTO


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CONCEPT OF PARTITION

COEFFICIENT

Effect of Silica


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100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

226 220 200 180 160 140 120 100 50 40 30

PRESSURE ( BAR)

PARTITION COEFFICIENT AT

DIFFERENT PRESSURES


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SOURCES OF IMPURITIES IN

STEAM

By evaporation from the boiler drum

By entrainment of boiler water droplets in

saturated steam.

As impurity present in feed water used in

desuper heater spray.


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EFFECTS OF SILICA

Silica has high partition coefficient, so it has

tendency to deposit from steam onto turbine.

Silica can deposit on turbine blades specially on

LP turbine, which can lead to significant loss ofoutput.


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THERE ARE TWO TYPES OF

BOILER WATER TREATMENT

USED AT VSTPS:


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HP DOSING

PHOSPHATE TREATMENTcombination of Di-Sodium Phosphate -

Trisodium phosphate is dosed in the drum

system. Phosphate will form a sludge withvarious ions & can be blown out throughCBD.

Na3PO4 + H2O Na2HPO4 + NaOH

Na2HPO4 + H2O NaH2PO4 + NaOH

NaOH +HCl (as impurity) NaCl + H2O


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Phosphate hide-out

NaH2PO4 is sparingly soluble at high temperatures(in fact the salt is closer to Na0.7H2.3PO4 )andtends to hideout leaving free NaOH in the solution

at high LOAD and reappears at low or ZeroLOAD.

Determination of free hydroxyl forms NaOH isnot accurate and there is natural tendency to addmore phosphate to make up that which is nolonger detectedrisk of building dangerous levelsof hidden out phosphate and Na carryover insteam


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HP DOSING

CAUSTIC TREATMENT

Where hide-out occurs or carry over into steam

results into unacceptable level of Na in the steam,

NaOH is temporarily dosed at times when chloride

target cannot be achieved


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CYCLE CHEMISTRY

Stage-I (210 MW) uni ts

AVTR+PHOSPHATE TREATMENT

Stage-I I & I I I (500 MW) units

AVTO+CAUSTIC TREATMENT

St I (210 MW) it


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SPECIFICATION OF 500 MW BOILER WATER:---1. Make Up Water:pH :6.8 7.2

Sp. Conductivity:


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BOILER WATER TREATMENT

3. BOILER FEED WATER:pH : 9.2 9.6

Sp. Conductivity:


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BOILER WATER TREATMENT5. SATURATED STEAM(SS)/SUPER HEATED(MS) WATER:

pH : 9.0 9.2

Sp. Conductivity:


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CHEMICAL CONTROL OF GENERATOR

STATOR WATER COOLING CIRCUITS

GENERATOR STATOR COOLANT ( D.M.WATER)

STATOR WATER PARAMETER TO BEMAINTAINED

- pH = 6.8 + .2 / 8.5 to 9.5 pH

- K < 1.0 s/cm- Copper < 50 ppb

- D.O. < 10 ppb in low D.O. regime

2.0 to 5.0 ppm in high D.O.

re ime


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Gen. O/L

Make-up

DMW

Coolers

MagneticFilters

NaOH

Dosing

IE

Gen. I/L

COND.

Fiters

PRIMARY WATER TREATMENT


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CORROSION OF COPPER

THERE ARE TWO METHODS WHERE THE

COPPER IS BEST PASSIVATED ANDMINIMUM CORROSION IS FOUND.

1. LOW DISSOLVED OXYGEN REGIME.

2. HIGH DISSOLVED OXYGEN REGIME.


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LOW D.O. REGIME

- Limits below 10 ppb

With low D.O. concentration, copper corrosion

is inhibited by a passive film of Cuprous

Oxide(Cu2

O).


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HIGH D.O. REGIME

- Limits 2 to 5 ppm

WITH HIGH D.O. CONCENTRATION,

COPPER CORROSION IS INHIBITED BY A

PASSIVE FILM OF CUPRIC OXIDE (CuO).

CW SYSTEM


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CW SYSTEM

Heat is dissipated from condenser ,by circulation of

cooling water. We are having open recirculation type CWsystem.As the circulating water absorbs heat from the

condenser & dissipates the heat in cooling tower, only the

pure water evaporates leaving the salts behind.Hence over

a period of time the circulating water gets concentrated.This is expressed in terms of COC (Cycle of

Concentration). COC is the ratio of concentration of

reference ion in the system to concentration of reference

ion in raw water.If Concentration of Mg in CW is 100PPM & that in raw in 50 PPM. The COC is 2.Similarly

the COC can be determined by knowing the concentration

of Magnesium, Chloride & SiO2 etc.


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CW SYSTEM

As the COC increases, the concentration of

salts & organic matter increases.This severelyenhances the scaling & bio-foulingtendency.In general the solubility increaseswith increase of temperature.

In case of Calcium Carbonate solubilitydecreases with increase of temperature. & tendto deposit as scales in tubes even at 35 Deg

Dosing of Chlorine as a biocide is done tocontain bio-fouling.However at higher pH itseffectiveness as biocide is reduced. It is max at

pH ~ 7- 8.


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Effluent Treatment Plant

To conserve water & meet the PCB norms on effluent

discharge the effluent treatment plant was brought up at

RSTPS.As our raw water is having high Calcium content, the

coal in this area is also having high Calcium content.Due to

this ash water pH is 11-12. Ash water cannot be used be

directly for further handling of ash because it is already

saturated with Calcium. There is a need for treatment of this ash water.In effluent

treatment plant water from CHP area, Ash pond overflow &

main plant effluent is treated .

Effluent of CHP is led to Coal Slurry settling pond (CSS)where coal particles settle.The clear water is taken to Central

Monitoring bay where main plant drain mixes.This water is

pumped to clarifier I/l where ash water comes by gravity

from ash pond.In the reactive clarifier water gets clarified.

E i t l P t


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Environmental Parameters Stack Emission: As per the latest consent order of APPCB,

the limits are:

SPM: 115 mg/NM3 (earlier 150 mg/NM3 )

SOx & NOx : No limit specified. However chimney height

225 Mtrs is specified for greater dispersion & distribution of

emission.

Ambient air: As per the latest consent order of APPCB, thepresent limits are as per residential area norms instead of

industrial area norms earlier:

SPM: 200 ug/M3 (Earlier 500 ug/M3 )

RPM:100 ug/M3 (Earlier no limit specified).

SOx :80 ug/M3 (Earlier 120 ug/M3 )

NOx: 80 ug/M3 (Earlier 120 ug/M3 )

* Fortnight monitoring in-house is being done for all these

parameters


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CHEMISTRY QUESTIONSQ1. What is the limit of SPM in stack emission in 500 MW unit.

a). 100 mg/Nm3 b). 150 mg/Nm3c)80 mg/Nm3 d)60 mg/ Nm3

Q2. Ammonia is dosed in fed water, this is called

a. HP Dosing b. LP dosing

C. HP-LP bypass dosing d. No dosing

Q3. Boiler water pH recommended for 500 MW unit is

a. 9.0 to 9.2 b. 9.2 to 9.6

C. 8.8 to 9.2 d. 6.8 to 7.2

Q4. Phosphate treatment is a

a. Boiler water treatment b. Feed water treatment

c. Raw water treatment d. none of these

Q5. Main function of CPU is toa. To remove ionic impurities b.To increase ph of water

c. to store water d. none of these


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RECOMMENDATIONS TO IMPROVE FEED

WATER QUALITY

Protection of DM water storage tank from dust and co2contamination.

Inspection of DM storage tanks at regular intervals.

Addition of make up water to condenser in spray form .

Air removal in condenser by air ejectors and vacuum pumps

of adequate capacity.


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Installation of magnetic filters/some other arrangements

to remove oxides from feed water.

Proper control of nh3 dosing in condensate.

Proper preservation of deaer-ator and boiler when laid off.

Restricting drum pressure during start of so that there is

no attemperation with fouled feed water.

Efficient cleaning of boilers and passivation to get prote-

ctive layer of adequate nature.

Efficient on-line monitoring of chemical parameters.


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FOR NORMAL TYPE-A CHEMISTRY

(WHERE THE ORP IS TYPICALLY

LESS THAN -300 MV) THE LEVEL OF

FEED WATER CORROSION

PRODUCTS WILL BE LESS THAN 10

PPB.

BUT UNDER CONDITION OF B AND C

CHEMICAL REGIME, SINCE ORP ISGREATER THAN ZERO THIS

FAVOURS THE GROWTH OF FeOOH .


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.REDUCES THE OVERALL CORROSION

RATE BECAUSE THE DIFFUSION OFOXYGEN TO THE BASE METAL IS

RESTRICTED.

FROM FAC POINT OF VIEW, THIS (FeOOH)

LAYER DISSOLVES MUCH SLOWLY AS

COMPARED TO MAGNETITE LAYER INTO

THE FLOWING FEED WATER, UNDER

IDENTICAL HYDRODYNAMIC CONDITIONTHAT EXISTED WITH TYPE - A

CHEMISTRY.

IT HAS BEEN OBSERVED THAT MOST OF


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IT HAS BEEN OBSERVED THAT MOST OF

THE FAC FAILURES IN FOSSIL FUEL FIRED

PLANTS HAVE OCCURRED WHEN ALL THE

TUBING (BOTH LP AND HP) IS STAINLESS

STEEL, AND THE CHEMISTRY IS TYPE A

(NORMAL AVT WITH HYDRAZINE

PRODUCING ORP


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ANOTHER AREA VULNERABLE TO FAC IS

HEATER DRAIN LINES IN BOTH ALL-

FERROUS AND MIXED-METALLURGY

SYSTEMS. HERE AGAIN THE MECHANISM

INVOLVED IS DISSOLUTION OF THE

SURFACE LAYERS OF MAGNETITE UNDER

REDUCING CONDITIONS. MOST HEATERS

IN PLANT OPERATION ARE VENTED,

WHICH MEANS THAT BOTH pH ANDOXYGEN LEVELS IN THE DRAIN LINES ARE

LOWER THAN IN THE FEED WATER.

THESE CONDITIONS EXACERBATE FAC.


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THE RATE OF METAL LOSS (WEAR RATE) IS

DEPENDENT ON:

FLUID TEMPERATURE,

MASS TRANSFER,

ALLOY COMPOSITION,

OXIDIZING/REDUCING POTENTIAL (ORP,RELATED TO DISSOLVED OXYGEN AND

REDUCING AGENT),

FLUID pH,

COMPONENT GEOMETRY,

UPSTREAM INFLUENCES AND STEAM

QUALITY.


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ALLOY CONTENT IS IMPORTANT

THAT EVEN TRACE AMOUNT OF

CHROMIUM, COPPER, AND MOLYBDENUM

CAN SIGNIFICANTLY REDUCE THE

SOLUBILITY OF THE OXIDE LAYER. MASSTRANSFER IS RELATED TO THE BULK

FLUID VELOCITY AND IS ALMOST LINEARLY

PROPORTIONAL TO THE RATE OF FAC.

THE SOLUBILITY OF THE LAYER IS

INVERSELY PROPORTIONAL TO LOCAL PH

DISTRIBUTION RATIO BETWEEN


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DISTRIBUTION RATIO BETWEEN

STEAM & BOILER WATER AT pH

9.5Drum Pressure Silica in Boiler Water

194 Kg/Cm2 130 ppb176 Kg/Cm2 220 ppb

159 Kg/Cm2 290 ppb

134 Kg/Cm2 500 ppb

117 Kg/Cm2 1000 ppb100 Kg/Cm2 2220 ppb

65 Kg/Cm2 4000 ppb

-- Boiler Drum Pressure is to be maintained so as

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